摘要:

We review available data and theories on the size and nature of interior power sources in the Jovian planets. Broad band infrared measurements indicate that Jupiter and Saturn have interior heat fluxes about 150 and 50 times larger, respectively, than the terrestrial value. While Neptune has a modest heat flux (∼5 times terrestrial), it is clearly detected by earth‐based measurements. Only Uranus seems to lack a detectable interior heat flow. Various models, ranging from simple cooling to gravitational layering to radioactivity, are discussed. Current evidence seems to favor a cooling model in which the escape of heat is regulated by the atmosphere. This model seems capable of explaining phenomena such as the uniformity of effective temperature over Jupiter's surface and the different emission rates of Uranus and Neptune. In such a model the heat radiated from the atmosphere may derive from depletion of a thermal reservoir in the interior, or it may derive from separation of chemical elements during formation of a core. Calculations indicate that in the earlier stages of cooling, Jupiter and Saturn may have more homogeneous abundances of hydrogen and helium and radiate energy derived from simple cooling. At a subsequent phase (which may be later than the present time), hydrogen and helium will separate and supply gravitational energy. Either model is consistent with a hot, high‐luminosity origin for the Jovian pl

ISSN:8755-1209    

DOI:10.1029/RG018i001p00001

年代:1980

数据来源: WILEY

摘要:

Abundances of trace elements in extrusive igneous rocks may be used as petrological and geochemical probes of the source regions of the rocks if differentiation processes, partition coefficients, phase equilibria, and initial concentrations in the source region are known. The characteristic trace element signature that each mineral in the source region imparts on the magma forms the conceptual basis for trace element modeling. The task of the trace element geochemist is to solve mathematically the inverse problem. Given trace element abundances in a magma, what is the mode of its source region? The most successful modeling has been performed for small planetary bodies which underwent relatively simple igneous differentiation events. An example is the eucrite parent body, a planet which produced basalts at ⋍4.6 Gy. and has been quiescent ever since. This simple differentiation history permits the calculation of its bulk composition (a feldspathic peridotite) and has led to the tentative identification of asteroid 4 Vesta as the eucrite parent body. The differentiation of iron meteorite groups in parent body cores is amenable to similar treatment. Quantitative calculations are currently hampered by the paucity of experimentally determined solid metal/liquid metal partition coefficients. The ‘anomalous’ behavior of Cr, however, suggests that IIIA, B irons and main group pallasites equilibrated with troilite, spinel, ferromagnesian silicates, or some combination thereof. The moon has undergone more complex differentiation, and quantitative geochemical modeling is correspondingly more difficult. Nevertheless, modeling the two‐stage evolution of mare basalts raises the possibility that the primordial moon did not have chondritic relative abundances of such refractory elements as Ca, Al, U, and the rare‐earth elements. The nonchondritic element ratios are characteristic of planetary, not nebular, fractionation processes and are consistent with the derivation of the moon from a precursor planet, possibly the earth. Alternatively, they may be artifacts of our inadequate understanding of differentiation in a deep, rapidly convecting magma ocean. The enormous complexity of terrestrial evolution over geologic time generally precludes quantitative geochemical modeling of the earth. For example, calculations investigating the petrogenesis of calc‐alkaline andesites are restricted to limiting the range of plausible hypotheses. In cases where Nd isotopic systematics indicate that the source region of a basalt suite had an approximately chondritic Sm/Nd ratio (e.g., Columbia River Plateau), semiquantitative calculations of the nature of the mantle may be possible. Geochemical modeling may have predictive value for an unsampled planet such as Mercury which may have had a relatively simple ther

ISSN:8755-1209    

DOI:10.1029/RG018i001p00011

年代:1980

数据来源: WILEY

摘要:

We present a review of the long‐wavelength gravity fields of the terrestrial planets, earth, moon, Mars, and Venus with particular emphasis placed on the interrelationship between gravity anomalies and tectonic processes. After first summarizing appropriate statistical formulas, we discuss the relevant continuum mechanical solutions for elastic, viscoelastic, and convecting media in terms of the relationship to the gravity field both for predicting gravity anomalies and for the use of gravity as a constraining boundary condition. Gravity data can provide a strong constraint for the first two classes of solutions, but a significant ambiguity exists in convection interpretations because of the competing effects from flow and boundary deformation. The question of finite strength of crustal and mantle materials plays a paramount role in gravity studies because of the possibility of model discrimination based on stress states. A variety of evidence is reviewed, and it is concluded that the finite strength of the terrestrial planets is unlikely to exceed a kilobar when subject to loads over long geologic periods. Isostasy plays a dominant role in any discussion of the lithospheric contribution to planetary gravity fields. Since it tends to minimize stresses, it is a basic state to be expected, and most useful discussions of the gravity fields are centered on departures from the isostatic state, either due to dynamic forces or to finite strength. For the earth the long‐wavelength anomalies arise largely from beneath the lithosphere and are related to mantle flow. Apart from the obvious correlation of gravity to the plate boundaries, the specific relationships between the gravity field and relevant flow parameters are complex, and there are at present no systematic relationships established between geophysical observables in the intraplate regions. On the moon, most of the contributions to the gravity field arise from the lithosphere and are supported by the finite strength of that layer. The mascons are due to a combination of several kilometer thick basalt layers and crustal collapse at the time of impact, forming ring structures and mantle uplift. Mars appears to be intermediate between the earth and moon and is dominated by the Tharsis anomaly, with the possibility that gravity anomalies arise from both lithospheric and sublithospheric regions. If the Tharsis Province is supported entirely by the lithosphere, then stress levels are of the order of 1 kbar, even with isostasy, which may imply partial dynamic support. Preliminary analysis of Venus gravity data suggests a power spectrum grossly similar to that of the earth, but systematic discrepancies among some harmonics may indicate differences in the nature of interior processes for the two planets. In particular, there appears to be a higher correlation of long‐wavelength gravity with topography for

ISSN:8755-1209    

DOI:10.1029/RG018i001p00027

年代:1980

数据来源: WILEY

摘要:

Planetary spacecraft have now probed the magnetic fields of all the terrestrial planets, the moon, Jupiter, and Saturn. These measurements reveal that dynamos are active in at least four of the planets, Mercury, the earth, Jupiter, and Saturn but that Venus and Mars appear to have at most only very weak planetary magnetic fields. The moon may have once possessed an internal dynamo, for the surface rocks are magnetized. The large satellites of the outer solar system are candidates for dynamo action in addition to the large planets themselves. Of these satellites the one most likely to generate its own internal magnetic field is Io.

ISSN:8755-1209    

DOI:10.1029/RG018i001p00077

年代:1980

数据来源: WILEY

摘要:

The volcanic filling of the lunar maria, the formation of mascons, the tectonic features of the mare regions, and the thermal evolution of the lunar lithosphere are all interlinked. To explore these processes for the major mascon maria (Serenitatis, Humorum, Imbrium, Orientale, Crisium, Nectaris, Smythii, and Grimaldi), we determine the characteristics and geometry of tectonic features in each mascon mare region, the distribution of major mare units and the flooding and subsidence history for each basin, the temporal relations between tectonic features and geologic units, the effective lithospheric thickness as a function of time in each mascon region, the implications of the derived thicknesses for lunar thermal structure, the relationship between local and global sources of stress in controlling lunar tectonic history, and the consequent limitations this relationship places on lunar thermal evolution.All mascon basins display some associated tectonic features: linear rilles, which are graben‐like features resulting from horizontal crustal tension, and/or mare ridges, which result from horizontal compression and buckling of near‐surface material. Crisium, Smythii, and Nectaris have ridges but no associated rilles. Mare ridges occur almost exclusively in mare basalt deposits. Where developed, rilles tend to be concentric to the basin and to occur outside the ridge systems, usually in the adjacent highlands. Volumes, sequences, and timing of basalt emplacement are determined from basin geometry and by stratigraphic reconstruction from remote sensing data and age determinations. Amounts of basin subsidence are determined from patterns of geologic units and present mare surface topography. Total volumes of basalt vary as a function of basin size, amount of subsidence, and stages of flooding (for example, Orientale, little flooding, and Imbrium, extensive flooding). The vast majority of filling for Serenitatis, Crisium, Nectaris, Imbrium, Humorum, and, possibly, Orientale apparently occurred early (3.8–3.6 b.y.). Lesser amounts were added to Serenitatis, Crisium, Imbrium, and Humorum at 3.6–3.2 b.y. Minor amounts were subsequently added to Serenitatis, Imbrium, and Crisium. The earliest basalts subsided shortly after their emplacement, indicating that the mare basalt load was superisostatic at that time. Linear rille formation appears to be restricted to the period prior to about 3.6 ± 0.2 b.y., although subsidence continued well past that time. Mare ridges occur throughout mare regions. Both subsidence and mare ridge formation must have continued until after emplacement of even the youngest mare basalt units.Detailed models of the flexural response of the lunar lithosphere to the mare basalt loads at the times of emplacement of major mare units permit evaluation of the effective thicknessTof the elastic lithosphere. The distribution of well‐developed graben concentric to each mare basin center is matched by a spatially variable thickness of the elastic lithosphere during the time of rille formation 3.6–3.8 b.y. ago:T≲ 25 km for Grimaldi;T= 40–50 km for Serenitatis, Orientale, and Humorum;T≃ 50–75 km for Imbrium; and T>75 km for Nectaris, Smythii, and Crisium. The distribution of mare ridges and the topographic relief of present mare surfaces are matched by a greater lithospheric thickness (T∼ 100 km) at the time of emplacement of the youngest mare units for most mare basins, and the required spatial variation in lithospheric thickness at ∼3.0 b.y. may be less than at ∼3.6 b.y. The growth of the lunar lithosphere beneath each mare basin is a natural consequence of the cooling of the outer portions of the moon. The global synchronism for the cessation of linear rille formation can be explained as being due to the superposition onto the local stress of a global thermal stress that shifted from extensional to compressional as the moon changed from net expansion to net contraction at or before 3.6 ± 0.2 b.y. ago.A pronounced spatial variation in effective lithosphere thickness during the time of early mare volcanism is clearly indicated. This variation is not simply the product of the average lunar thermal evolution, nor is it explainable solely on the basis of differences in mare fill age or in basin size or basin age. These variations likely represented large‐scale inhomogeneities in the thermal structure of the lunar crust and uppermost mantle, arising from lateral variations in crustal heat sources, crustal heat transport, or sublithospheric heat flow. These inhomogeneities apparently lessened in magnitude with time as thermal variations were smoothed o

ISSN:8755-1209    

DOI:10.1029/RG018i001p00107

年代:1980

数据来源: WILEY

摘要:

A survey of published descriptions of 32 of the largest, least eroded terrestrial impact structures reveals that the amount of melt at craters in crystalline rocks is approximately 2 orders of magnitude greater than at craters in sedimentary rocks. In this paper we present a model for the impact process and examine whether this difference in melt abundance is due to differences in the amount of melt generated in various target materials or due to differences in the fate of the melt during late stages of the impact. The model consists of a theoretical part for the early stages of impact, based on a Birch‐Murnaghan equation of state, a penetration scheme after Shoemaker (1963), and an attenuation model modified from Gault and Heitowit (1963), and a descriptive part for the later stages of impact, based on field observations at the large terrestrial craters. The impacts of iron, stone, permafrost, and ice meteorites 1 km in diameter into crystalline, carbonate, dry sandstone, ice‐saturated sand, and ice targets are modeled for velocities of 6.25, 17, and 24.6 km/s. Tables of calculated crater volume, depth of penetration of the meteorite, equivalent scaled depth of burst, radii to various peak pressure isobars, volume of silicate melt, and volume of water vapor (or, in the case of carbonate, carbon dioxide vapor) are presented. Simple algebraic expressions for pressure attenuation are derived: for the near field,dX/dR= 3Xn/R(1 ‐n), whereXis the pressure normalized to an averaged bulk modulus for the target rocks,Ris the radius normalized to the radius of the cavity in which energy is initially deposited, andnis the pressure derivative of the bulk modulus. For the far field the pressure attenuation is given bydX/dR∼−3X/R. For most materials considered,n= 4–6, and therefore the near‐field attenuation is proportional toR−3.65‐R−4and the far‐field attenuation is proportional toR−3. The calculations show that the volume of material shocked to pressures sufficient for melting should not be significantly different in sedimentary and crystalline rocks. Hence we conclude that shock melt is formed in the early stages of the cratering process by impacts into rocks rich in volatiles but is destroyed by the cratering process. We propose that the melt is finely dispersed by the great expansion of shocked volatiles upon release from high pressure and that suevite units are the product of this process. The fragmented silicates produced by this process may react penecontemporaneously with the hot volatiles to produce hydrated minerals such as clays. This process may produce hydrothermally altered minerals in planetary regoliths, such as the Martian regolith. The dispersion of shock melt by volatile expansion may also account for the apparent lack of lunarlike melt sheets on the surface of Mars. Because large amounts of volatiles vaporize during impact and are transferred from depth either into space, into the atmosphere, or onto near‐surface ejecta by condensation, repeated impact degasses a planet, depleting some layers in volatiles and, unless the volatiles escape the

ISSN:8755-1209    

DOI:10.1029/RG018i001p00143

年代:1980

数据来源: WILEY

摘要:

This review summarizes some of the recent results on the disturbed upper atmosphere obtained by satellite‐borne gas analyzers. According to these measurements, magnetospheric activity leads to the development of two different disturbance zones. The high‐ and middle‐latitude region is characterized by an increase in the heavier constituents Ar, N2, and O2, by a height‐dependent behavior of O, and by a significant decrease of He. The reaction time of the atmosphere is much smaller than one orbital period. At lower latitudes a moderate increase of all constituents is observed. A comparison between atmospheric and ionospheric data demonstrates that, in contrast to positive effects, negative ionospheric storms are closely coupled to changes in the neutral composition. In addition, model calculations fully support a causal relation between both phenomena. Given this correlation, atmospheric and negative ionospheric disturbance effects have certain variations in common. These include systematic changes with the magnetic storm intensity, with magnetic position, with local time, and with season. Whereas the presently available empirical and theoretical models are quite capable of reproducing the basic properties of the observed atmospheric perturbations, these algorithms are not sophisticated enough for a more detailed description of t

ISSN:8755-1209    

DOI:10.1029/RG018i001p00183

年代:1980

数据来源: WILEY

摘要:

Considerable important research in upper atmosphere geophysics is carried out through the use of arrays of ground‐based magnetometers. In order to better delineate the ionospheric and magnetospheric currents and waves as measured by these arrays, it is important to understand the conductivity of the earth's structure under the individual stations. Geomagnetic depth sounding studies are used to deduce the earth's conductivity profiles. In most studies, ‘induction arrows,’ or ‘induction vectors,’ are plotted on maps for graphical representations of lateral inhomogeneities in underground conductivity structures. Different methodologies and different arrow conventions have been used by a number of authors for deriving these vectors, often without relating their techniques to other work in the field. We review herein the various methodologies (except transfer functions) and present a unifying picture to the representations that should prove useful to researchers in both space physics and solid eart

ISSN:8755-1209    

DOI:10.1029/RG018i001p00203

年代:1980

数据来源: WILEY

摘要:

A theoretical discussion is presented of the propagation, stability, generation, scattering, interactions, and dissipation of low‐frequency waves trapped along continental shelves. Brief accounts of laboratory and oceanic evidence of these ‘shelf’ waves are also given. The emphasis is on those aspects of shelf wave dynamics that have been published during the past 5 years. Several theoretical and observational questions that deserve attention in the future are r

ISSN:8755-1209    

DOI:10.1029/RG018i001p00211

年代:1980

数据来源: WILEY

摘要:

This review article highlights the three‐century development of our scientific understanding of ocean tides, culminating through myths, paradoxes, and controversies in a global tide model that now permits the prediction of the instantaneous total tide anywhere in the open oceans with an accuracy of better than 10 cm. All major aspects of tidal research, including empirical, mathematical, and empirical‐mathematical methods, are considered. Particular attention is drawn to the most recently developed computerized techniques comprehending hydrodynamical dissipation and secondary tide‐generating forces, finite‐differencing schemes, geometric boundary and bathymetry modeling, and hydrodynamical interpolation of properly selected empirical tide data. Numerous computer experiments are mentioned that were carried out by various researchers in order to evaluate the magnitudes of the featured effects. Further possible improvements are mentioned, especially in nearshore areas, in the Arctic Sea, and near Antarctica, where empirical tide and bathymetry data are either rough or m

ISSN:8755-1209    

DOI:10.1029/RG018i001p00243

年代:1980

数据来源: WILEY